Thermal inkjet hardcopy devices such as printers, graphics plotters, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the media.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). The printhead has an array of ink ejection elements formed in the substrate. The printhead incorporates an array of ink ejection chambers defined by a barrier layer formed on the substrate. Within each ink ejection chamber is the ink ejection element formed in the substrate. Precisely formed orifices or nozzles formed in a nozzle member is attached to a printhead. Each ink ejection chamber and ink ejection element is located opposite the nozzle so that ink can collect between it and the nozzle. The ink ejection chambers receive liquid ink from an ink reservoir. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the ink ejection elements. When electric printing pulses activate the inkjet ink ejection element, a droplet of ink is ejected from the printhead. Properly sequencing the operation of each ink ejection element causes characters or images to be printed upon the media as the printhead moves past the media.
The ink cartridge containing the printhead is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After all such completed movements, the medium is advanced forward and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
One problem with conventional inkjet printers is droplet or dot displacement. This problem is most apparent when printing a vertical line. Typical print cartridges cycle through their firing order only once per pixel. Since print cartridges continuously proceed through their firing order as the scanning carriage moves across the medium, ink droplets ejected from nozzles at the beginning of the firing order are deposited at their desired location, while those ejected at the end of the firing order are displaced from their desired position by a distance approximately equal to the pixel width. For a 600 dpi printer this error distance is 42 microns. Thus, a resulting vertical line will appear jagged rather than straight.
One solution to the dot displacement problem is to stagger the physical position of the nozzles and their respective ink ejection chambers on the substrate of the printhead. Although effective at solving the dot displacement problem, this approach is relatively complex. The ink flow distance from the edge of the substrate to an ink ejection chamber varies depending on the location of the particular ink ejection chamber. Ink ejection chambers located closer to the edge refill faster than those further away. This creates differences in both the volume and velocity of ejected ink droplets.
Another solution to the dot displacement problem involves rotating the entire printhead. This approach, however, employs a more complex print cartridge and scanning carriage in order to create the rotation. In addition, this print cartridge is more difficult to code and requires additional memory, since data for many different columns must be buffered up simultaneously.
Still another approach is minimizing dot displacement error by increasing the number of times per pixel that a print cartridge with non-staggered nozzles cycles through its firing order. These high firing frequency, multi-drop per pixel print cartridges can be designed with no ink ejection element stagger and no rotation of the printhead, because the total positional error produced is normally small, i.e., a fraction of a column width. This design gives the advantage of having the fluidic responses of the firing chambers all the same, which results in faster print cartridges with less overshoot and puddling. However, even the small positional errors can become visible defects when they are repeated in a regular pattern.
Therefore, there is a need for a simple, high speed printer that reduces dot displacement error without ejection element stagger or rotation of the printhead.